Multivibrator with electrically variable pulse repetition frequency



1966 A. E. ENGLUND, JR., ETAL 3,293,569

MULTIVIBRATOR WITH ELECTRICALLY VARIABLE PULSE REPETITION FREQUENCYFiled Jan. 4, 1 965 2 Sheets-Sheet 1 lNVENTORS ARVID E. ENGLUNDNJR.

WILLIAM J. LITTILE BY WNW THEIR ATTORNEY.

United States Patent "ice 3,293,569 MULTKVHBRATUR WTTH ELECTRTCALLYVARI- AlllSLlE PUISE REPETETIUN FREQUENCY Arvid E. England, lira, andWilliam J. Little, Lynchburg,

Va, assignors to General Electric Company, a corporation of New YorkFiled Jan. 4, 1965, Ser. No. 423,164 ll Claim. (Cl. 331113} Thisinvention relates to a pulse generator and, more particularly, to afree-running rnultivibrator which is electrically controlled to vary thepulse repetition frequency.

Free-running or astable multivibrators are well-known pulse generatorsconsisting of a pair of conducting devices which are cross-coupledthrough an RC circuit to drive the devices alternately into conduction.The RC network, which cross-couples the conductive devices, performs atiming function and normally sets the rate at which the conductingstates of the devices are switched and, hence, controls the repetitionrate of the pulses generated by the multivibrator. Customarily, once themultivibrator has been designed and the values of the RC components aredetermined, the repetition rate of the multivibrator is fixed.

A need exists in certain applications for pulse generators of the multivibrator type which may be electrically controlled to vary therepetition rate of the output pulses. Some circuit arrangement must,there-fore, be found for electrically varying the values of thecomponents in the RC timing circuit that control the rate at which theconductive devices are switched.

It is, therefore, a primary object of this invention to provide anelectrically controlled variable speed multivibrator.

Another object of this invention is to provide a freerunningrnultivibrator wherein the time constant of the timing circuit iselectrically varied to control the repetition frequency of the outputpulses from the multivibrator.

Still another object of this invention is to provide a multivibratorwherein components of the timing circuit are selectively switched intoand out of the multivibrator timing circuit in response to a controlsignal to vary the repetition rate of the output pulses from themultivibrator.

Other objects and advantages of the invention will become apparent asthe description thereof proceeds.

The various objects and advantages of the instant invention are realizedby providing a free-running rnultivibrator in which the resistance ofthe multivibrator RC timing circuit is electrically controlled to changethe time constant of the device. The resistance is varied by providingone or more additional resistors in shunt with the main or primaryresistor of the timing circuit. These additional resistors are eachconnected in series with a diode, and the conductive state of the diodesis controlled in response to an external control signal to selectivelyconnect or disconnect the additional resistors thereby varying theequivalent resistance of the resistor of the multivitions at the variouspoints in the multivibrator circuit of FIG. 1 and which are useful inunderstanding the operation of the multivibrator.

3,2935% Patented Dec. 2Q, 19%6 FIG. 1 illustrates a variable speedmultivibrator con structed in accordance with the instant inventionwherein the resistance of the multivibrator RC timing circuit iselectrically controlled to vary the switching speed of the multivibratorand, hence, the repetition rate of the output pulses. The.multivibrator, shown generally at l, consists of two NPN transistors Qand Q each transistor being coupled from its collector to the base ofthe other transistor through a timing network. Transistors Q and Qinclude base electrodes 2 and T collector-electrodes 4 and 5, andemitter electrodes 6 and 7. The base electrodes are each cross-coupledto the alternate collector through the coupling capacitors 8 and 9 whichform part of the timing circuit of the transistor. The transistor basesare returned to a positive supply bus +13 through resistors 10 and illwhich, with capacitors 3 and 9, form the primary components of themultivibrator timing circuit. Operating potential for the transistors isprovided by connecting collectors 4 and 5 to the positive supply bus +Ethrough the collector-resistors i3 and 1d, and emitters 6 and 7 to asource of reference voltage, such as ground, through the common emitterresist-or 15 which is by-passe-d for AC. by capacitor 16. An outputterminal 17 is connected to collector 5 of transistor Q As thus fardescribed, multivibrator it is a perfectly conventional free-runningmu-ltivibrator with the switchirrg rate between transistors Q and Qcontrolled respectively by the RC time constants of resistor ill andcapacitor 8, and resistor 11 and capacitor 9. The transistors conductalternately and are driven into saturation whenever the base of thetransistor becomes more positive than its associated emitter, with thevoltage at point A in the emitter circuit being established by thevoltage drop across emitter-resistor 15. The base return and timingresistors it) and 11 are substantially larger than collector-resistors13 and it so that the capacitor connected to the collector of thenonconducting transistor charges rapidly to the supply voltage +Ewhereas the capacitor connected to the collector of the saturated orconducting transistor, which has previously been charged to E dischargesat a much slower rate through its associated base return resistor.

To vary the switching speed of the multivibrator, and hence therepetition rate of the output pulses at terminal 17, an alternatenetwork is connected to the transistor bases, shown generally at 20. Thenetwork includes additional timing resistors 21 and 22 connected inseries with diodes 23 and 2 3, respectively. The series combination ofresistor 21 and diode 23 is connected to the junction of base 2 andcapacitor 8, and resistor 22 and diode 24 are connected in series to thejunction of capacitor 9 and base 3.

Diodes 23 and 24 are selectively biased into conduction to connectresistors 21 and 22 in shunt with resistors Ill and 11 to vary theequivalent resistance and thereby the time constant of the timingcircuit and the operating speed of the multivibrator. To this end, acontrol circuit 25 is provided for diodes 23 and 24 which selectivelybiases these diodes into conduction. Circuit 25 includes an NPNtransistor switch Q which has a base 26, a collector 27, and an emitter28. Control signals at terminal 31 are coupled to base 26 throughresistor 30 and control the conductive state of transistor Q and therebydiodes 23 and 24. Base at is returned to ground by base return resistor32. The emitter voltage V at point B in the emitter circuit, is fixed ata value established by the reverse breakdown voltage of a voltagereference device, such as a Zener diode 33, connected between emitter 23and ground and dropping resistor 3 connected to the positive bus EVoltage V is positive with respect to ground but is less positive thanvoltage V at point A in 3 the emitter circuit of the mutltivibrator toobtain positive switching of the diodes. Operating potential forcollector 27 is obtained by means of collector-resistor 35 connected thej-E bus.

The voltage V at point C in the collector circuit of "Q controls diodes23 and 24, the anodes of which are connected to this point by commonlead 36. A positive voltage at input terminal 31, which is greater thanthe positive voltage V at point B in the emitter circuit, forward-biasesthe base-emitter junction of Q and switches the transistor intosaturation. The voltage V at the collector is thereby held essentiallyat the value of the emitter reference voltage V and diodes 23 and 24 arenot forward-biased. Whenever the voltage at input terminal 31 dropsbelow the emitter reference voltage V the base emitter junction of Q isreverse-biased, driving Q to cut-oft, so that the collector voltage Vrises essentially to the value at the E bus. Whenever the voltage atpoint C rises to B diodes 23 and 24 are forward-biased, and resistors 21and 22 are connected in shunt with the main timing resistors lit and 12through diodes 23 and 24, lead 36 and Q collector-resistor 35. Theequivalent resistance of the multivibrator timing circuit is reducedincreasing the switching speed of the multivibrator and the repetitionrate of the output pulses at terminal 17. Thus, the speed of themultivibrator is controlled by an electrical control signal, and therepetition rate of the output pulses is correspondingly varied.

The manner in which the multivibrator is electrically controlled to varyits speed and the output pulse rate may best be understood in connectionwith the diagrams of FIGS. 27 which represent the wave forms at thebases and collectors of transistors Q Q and Q Assume that at some timei=0, transistor Q is in the conducting state, and transistor Q is in thenonconducting state. Also, a sufliciently positive potential is presentat input terminal 31 o that Q is in saturation, and the collectorvoltage V is equal to V and multivibrator 1 runs in its slow mode. Withtransistor Q conducting and in saturation, the collector-emitterresistance of the transistor is very low, and the voltage drop acrossthe transistor is so small that the voltage at the collector isessentially equal to the voltage V at the emitter. Curve 35 of FIG. 2shows the voltage V Q at the collector of Q as equal to V The voltage atthe collector of transistor Q V Q in FIG. 4, on the other hand, isapproximately at +E since Q is cut off, and the voltage drop acrosscollector-resistor 13 is very small. The potential at base 3 oftransistor Q2, V Q is more positive than the emitter voltage V as shownby curve 37 of FIG. 3, maintaining Q in the saturated state. During thetime interval i=0 to t=t the potential V Q at base 2 of Q has beenslowly rising as capacitor 8 discharges through resistor 10, as shown bycurve 38 of FIG. 5.

Assume that at t=t V Q has risen sufficiently so that the base of Q ismore positive than the emitter potential V Q is then driven intoconduction, and Q is driven to cut-off. As Q is cut off, the voltage atcollector rises from V the value of emitter voltage, toward the voltageat the positive bus E Capacitor t charges to the voltage of E with thepolarity shown at a rate determined by the time constant of resistor 14and capacitor 8. Since, as pointed out previously, the value ofcollector-resistor 14 is very small compared to the value of the primaryresistors, the rate at which capacitor 8 charges and at which thevoltage at the collector 5 rises to E is very rapid and is representedby the portion of curve 35 of FIG. 2 designated R C During the intervalthat Q was cut off, capacitor 9 charged up to the full supply voltage Ewith the polarity shown in FIG. 1. The moment Q is driven intoconduction, the voltage at collector 4 of Q drops from E to the voltageV at its emitter. Since capacitor 9 cannot discharge instantaneously,the only path being through timing resistor 11, base 3 of collector Q isinstantaneously driven negative. Q at +V the voltage at the base of Qbecomes IV -E As E is larger than V it can be seen from curve 37 of FIG.3 that at r=r base 3 is driven substantially more negative than itsemitter, and Q is cut off. Capacitor 9 now begins to discharge throughthe main timing resistor 11. Initially, diode 24 is also driven intoconduction to permit partial discharge of capacitor 9 through alternateresist-or 22 and Q collect-orresistor 35 as well.

It will be recalled that with a positive control signal at terminal 31,the voltage V at the collector of Q is positive, as may be seen fromcurve 41 of FIG. 7, but less positive than the voltage V at point A inthe multivibrator emitter circuit. The anode of diode 24 is, therefore,at a positive voltage with respect to ground. The cathode is connectedto base 3 of Q which, as may be seen from FIG. 2, is negative withrespect to ground. Diode 24 is thus forward-biased and conducts, andcapacitor 9 discharges through resistors 11, 22, and 35, at a fairlyrapid rate, as shown in FIG. 3 by the steeply rising portion of curve 37between 2 and t At some point in time t capacitor 9 has dischargedsufficiently so that the potential at the junction of base 3 andresistor 22 is now equal to or slightly more positive than the voltage Vat the collector of Q Diode 24 is now reverse-biased, disconnectingresistor 22 from the discharge path an-d, hence, from the timing circuitof the multivibrator. Capacitor 9 now continues to discharge throughmain discharge resistor 11, the resistance of which is substantiallygreater than the equivalent resistance of resistors 11, 22, and 35 inshunt, so that the discharge rate is substantially reduced. The timeperiod required for the capacitor to discharge sufficiently for base 3to become more positive than the emitter voltage V is now determined bythe time constant C R which is substantially greater than before. Thismay be seen in FIG. 3 by the portion of curve 37 between t and t At tcapacitor 9 has discharged sufficiently so that base 3 of Q is now morepositive than V forward-biasing the base emitter junction and driving Qinto conduction and saturation. The collector of Q drops from E to V asmay be seen in FIG. 2. Simultaneously, Q is driven into cut-oil, and thevoltage at its collector, as shown in curve 36 of FIG. 2, rises from Vtowards E at a rate determined by the time constant C R ofcollector-resistor 13 and capacitor 9. Capacitor 9 similarly charges upto the value of the supply voltage E when Q is cut off.

In time interval from t to t during which Q has been cut off, capacitor8, of course, charges to the full value of the supply voltage E At twhen Q conducts, the potential V Q at the base of Q is driven negative,as shown in curve 38 of FIG. 5. This occurs because the voltage V Q atcollector 5 drops almost instantaneously from E to V while capacitor 8cannot discharge instantaneously. Consequently, base 2 of Q is drivennegative with respect to its emitter. Capacitor 8 now begins todischarge. From 1 to 1 diode 23 conducts since its cathode is morenegative than its anode, rapidly discharging capacitor 8 throughresistors 10, 21 and 35, until the voltage at the junction of base 2 andresistor 21 becomes more positive than the voltage V at the collector ofQ This reverse-biases the diode, disconnecting resistor 21 from thedischarge path, and capacitor 8 continues to discharge at a ratedetermined by the resistance of resistor 10. The slow discharge ofcapacitor 8 continues until at time 1 base 2 once more becomes morepositive than the emitter, and Q is driven into saturation, and Q is cutoff.

As long as the positive control voltage at input terminal 31 exceeds thevoltage V at the emitter of Q as shown With collector 4 of 5. by curve40 of FIG. 6, diodes 23 and'24'condi1ct only temporarily, and the rateat which the multivibrator operates is approximately that shown in FIG.2 by the pulse period T which represents the slow operating mode of themultivibrator. If at time the positive control voltage at input terminal31 is reduced to a value less than the emitter voltage V Q is cut offsince its base emitter junction is now reverse-biased. The voltage V apoint C in the collector circuit of Q rises from a value less than V toapproximately the value at the positive bus E With the voltage at pointC being VCEEBB, the anodes of diodes 23 and 24 are positive with respectto the cathodes during the entire operative cycle and additional timingresistors 21 and 22 are permanently connected in shunt with the maintiming resistors and 11 through the diodes and through Qcollector-resistor 35. The discharge time of capacitors 8 and 9 is nowdetermined by the equivalent resistance of the parallel combination ofresistors, which equivalent resistance is much lower than the resistanceof the resistors 11 and 12. Hence, as shown in FIGS. 3 and 5, thedischarge of capacitors 8 and 9 and the rate at which the base voltagesrise during intervals Z 4 t t Z 4 etc., is much more rapid. Theswitching time of the transistors Q and Q is reduced substantially, asmay be seen from curves 35 and 36 of FIGS. 2 and 4. The multivibratorperiod T is reduced, and the repetition rate of the output pulses fromthe multivibrator is increased. The multivibrator remains in this highspeed mode as long as the input signal at terminal 31 remains below V sothat the collector voltage V shown by curve 41 of FIG. 7, isapproximately at E Whenever the voltage at terminal 31 becomes morepositive than V the free-running multivibrator again reverts to a slowoperating mode, and the repetition rate of the output pulse is reduced.

It will be seen from this description and the wave forms of FIGS. 2 to 7that a simple and etfective arrangement for varying the operating speedof a free-running multivibrator has been provided by electricallycontrolling the time constant of the timing network of themultivibrator.

A variable speed multivibrator, such as the one shown in FIG. 1, wasconstructed and found to operate in the manner described. The followingis a tabulation of the component values of the multivibrator soconstructed. These values are given by way of example only and are notconsidered to be in any way limiting of the invention. The multivibratorthus constructed operated at a rate of approximately four cycles persecond in the slow running mode and approximately thirty cycles persecond in the fast running mode.

R =39O kilohms R =390 kilohms R =8.2 kilohms R14=8.2 R =3.9 kilohms R=56 kilohms C =.5 farad R =56 kilohms C farads Q Q Q =19A1l5123-1,similar to 2N2712 Diodes 23, 24=19A115250-L silicon diode Zener diode33:4036887-2, Zener diode 2.8 v. E =+l0 volts R =10 kilohms R =30kilohms R34:3.3 R =7.5 kilohms C =.5 farad primary timing resistors 10and 11 in order to permit the capacitors 8 and 9 to charge rapidly tothe supply voltage when their associated transistor is cut. off.Furthermore, the resistance of primary resistors 11 and 12 should besmall enough to assure that the transistors are saturated when they areconducting. Additional timing resistors 21 and 22 are, of course, chosento have values such that the equivalent resistance of the network withthe resistors in shunt produces the desired speed of operation.

One additional advantage flowing from the use of diodes 23 and 24 in theslow operating mode is that the diodes also function to protect thetransistor against excessive base emitter reverse voltages. It will berecalled that in the slow operating mode when a positive voltage isapplied to input terminal 31, one or the other of diodes 23 and 24 isdriven into conduction for at least part of the operation cycle toprovide rapid discharge of the capacitor until the voltage at thetransistor base is more positive than the voltage V at the collector ofQ This insures that when the base of the nonconducting transistor isdriven negative, that the reverse-bias is impressed across the baseemitter junction for a very short time only, thereby minimizingavalanche efiects or other damage to the transistor which may occur ifan excessively large reverse-biasing voltage is applied to the baseemitter for an excessive period of time. This is a problem which isparticularly significant for example with alloy junction transistors,and the customary technique to avoid this problem is to connectindividual diodes across the base emitter junction to limit thereverse-bias. In the instant circuit, the switching diodes 23 and 24provide this additional protective function by rapidly discharging anyreverse biasing voltage before it has an opportunity to do damage.

Although a number of specific embodiments of the invention have beenshown, it will, of course, be understood that the invention is notlimited thereto since many modifications, both in the instrumentalitiesand circuit arrangement employed, may be made. It is contemplated by theappended claim to cover any such modifications which fall within thetrue spirit and scope of this invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

In an electrically controlled variable-speed multivibrator for producingoutput pulse trains at two distinct repetition rates, the combinationcomprising:

(a) first and second transistors, each having collector,

emitter and base electrodes;

(b) timing circuit means for establishing regenerative feedback pathsbetween the collector of said first transistor and the base of saidsecond transistor, and the collector of said second transistor and thebase of said first transistor to establish the normal operating speed ofthe multivibrator for producing a pulse train having a first repetitionrate, said last named means including a capacitor connected between thecollector of said first transistor and the base of said secondtransistor, and a primary timing resistor, connected from the base ofsaid second transistor to a point of potential to provide a dischargepath for said first capacitor, and a further capacitor and a furtherprimary timing resistor connected respectively between the collector ofsaid second transistor and the base of said first transistor and thebase of said first transistor and said point of potential;

(0) two further timing resistors adapted to be connected in shunt withsaid primary timing resistors to vary the time constant of the timingcircuit means and the pulse repetition rate of the output pulses fromsaid multivibrator, and

(d) electrically activated switch means, including diodes connected inseries with said further resistors, said diodes conducting temporarily,whereby timing for the normal operational speed is establishedessentially by the time constant of said capacitors and primaryresistors,

(e) a third transistor having input, output, and common electrodes, aconstant voltage reference element connected between the commonelectrode and a point of reference potential, said third transistorbeing adapted to receive a control signal at its input electrode tochange its conducting state, means coupling the diodes to the outputelectrode of said third transistor whereby said diodes are continuallyforwardbiased in response to said control signal for continu allyconnecting said further resistors in shunt With the primary resistancefor the duration of said control signals, thereby varying the equivalentresistance of said timing circuit and the operating speed of themultivibrator to produce a pulse train having a different repetitionrate.

References Cited by the Examiner UNITED STATES PATENTS 3,129,391 4/1964Kabell 331-113 5 3,152,306 10/1964 Cooper et al 331-113 FOREIGN PATENTS1,245,754 10/ 1960 France. 1,139,876 11/1962 Germany.

10 OTHER REFERENCES I. Kyle, Electronics World, Electronic Sirens, pages32, 33, April 1964.

ROY LAKE, Primary Examiner.

J. KOMINSKI, Assistant Examiner.

